As is well known, fabricating semiconductor chips is a multi-step process. Briefly explained, silicon wafers, sliced from a crystal ingot, initially are lapped flat and polished to a mirror-like finish. A layer of single crystalline silicon is subsequently grown on each wafer and the wafers are oxidized at elevated temperatures approaching 1000.degree. C. A light-sensitive "photo-resist" coating may then be applied to each wafer and a wafer stepper used to expose the photo-resist coating. Exposing the coating produces multiple prints containing images of several integrated circuit chips on each wafer.
Following exposure, the photo-resist coatings are developed and baked to harden the patterned prints onto the silicon wafers. The wafers then contact a reactive gas discharge, etching exposed portions of the wafers, before having ionized Boron atoms or other impurities implanted into the patterns. A low temperature (350.degree. C.) plasma discharge deposits silicon dioxide on the wafers at low pressure, while circuit component contacts may be made by depositing onto the wafers a thin aluminum or similar metallic film. Each wafer layer is then cut into multiple semiconductor chips using a precision diamond saw and the chips are attached to packages having contact lead and wire connections. Finally, each chip is encapsulated in plastic for mechanical and environmental protection.
Because even microscopic airborne impurities can degrade the quality and yield of the fabricated chips, many of these manufacturing steps, including those of applying the photo-resist coating to the wafers and exposing the integrated circuit chip images on the coatings, are performed in facilities referred to as "clean rooms." The atmosphere of a clean room is generally regulated to limit the numbers and types of particles capable of contacting the silicon wafers. Humidity/ventilation/air conditioning (HVAC) equipment is used to condition the air within the clean room, in part to reduce particle concentrations resulting from other sources of contamination such as the wafer processing and handling machinery. Additionally, the bodies of workers operating in the clean room are conventionally enveloped by sterile clothing to prevent skin, hair and other personal particulate matter from being deposited on the wafers.
An average manufacturing facility may include as many as 200 pieces of processing and handling equipment for fabricating semiconductor chips. To accommodate both the various equipment used to process the wafers and the wafer-handling personnel, the size of many clean rooms frequently may approach 20,000 square foot. Such rooms are costly to construct, requiring sophisticated monitoring and air conditioning equipment to regulate, even moderately, the large-scale environments. Typically, within a clean room are placed multiple environmental control enclosures, each of which provides more rigorous decontamination standards within a work area to establish an environment within which today's chips may be fabricated. These environmental enclosures can consume a significant amount of energy since they have traditionally been designed to accomplish HVAC conditioning within the work area without accounting for the energy employed to accomplish this goal.
For example, a conventional environmental enclosure is disposed within a clean room so as to separate a clean room isle from a "core area" wherein machinery and piping resides to handle the fabrication process. The core area contains unconditioned air in terms of temperature, humidity and particles per cubic foot. The conventional environmental enclosure is designed to draw in unconditioned air from this core area, then filter and condition the air before moving the air into the work area of the environmental enclosure. To accomplish this, existing environmental enclosure apparatus can be expensive to operate, particularly since the enclosure is already disposed within a clean room.
As an alternate approach, reference the self-contained environmental enclosure system of U.S. Pat. No. 5,195,922, wherein a control system is described having a series of modules, connectable isolation chambers and associated atmospheric control equipment. This disclosure is put forth as an alternative to a traditional clean room. Although apparently achieving its goal of not requiring a clean room, such machinery is clearly complicated and expensive to operate.
Thus, there exists a need in the art for an improved environmental control apparatus which is designed with the goal of achieving enhanced energy efficiency, while still providing a work chamber that meets the necessary particulate and air conditioning requirements. The present invention addresses this need within the environment of a clean room.